Immunogens comprising capsular saccharide antigens conjugated to carrier proteins are well known in the art. Conjugation converts T-independent antigens into T-dependent antigens, thereby enhancing memory responses and allowing protective immunity to develop, and the prototype conjugate vaccine was for Haemophilus influenzae type b (Hib) [e.g. see chapter 14 of ref. 1]. Since the Hib vaccine, conjugated saccharide vaccines for protecting against Neisseria meningitidis (meningococcus) and against Streptococcus pneunioniae (pneumococcus) have been developed. Other organisms where conjugate vaccines are of interest are Streptococcus agalactiae (group B streptococcus), Pseudomonas aeruginosa and Staphylococcus aureus. 
Where saccharides are included in vaccines and other biological products then regulatory authorities generally require their characterisation. A common technique used for saccharide characterisation is anion chromatography, and in particular high performance anion exchange chromatography (HPAEC), usually with pulsed amperometric detection (PAD) [2,3]. Suitable HPAEC-PAD systems are provided by Dionex™ Corporation (Sunnyvale, Calif.) e.g. the BioLC™ system. These systems can quantitatively analyse individual saccharides within mixtures without the need for derivatisation or pre-analysis separation, and analysis of mixed saccharides can be used in saccharide profiling.
When analysing saccharides, the eluate from a HPAEC column is typically analysed using a pulsed amperometric detector (PAD) i.e. detection is based on electrical current. At suitable (high) pH, carbohydrates can be electrocatalytically oxidised at the surface of electrodes by applying a positive potential. The current generated is this way is proportional to the carbohydrate concentration, allowing detection and quantification of the carbohydrate by amperometry. Compared with simple amperometric detection, the PAD technique intersperses short pulses of a cleaning and regeneration potential with the standard detecting potential, thereby avoiding difficulties that arise when oxidation products of analytes foul the electrodes. As well as being used for HPAEC analysis, PAD is also used for analysing HP cation-exchange chromatography [4] and other HPLC separations.
To obtain further analytical information, particularly when dealing with compounds that are not amperometrically active and with chemically-modified compounds, the inventors decided to analyse eluates by spectroscopic means. Unfortunately, the high pH used during HPAEC analysis of capsular saccharides means that hydroxide ions are present in the eluate, and the high absorbance of these ions (particularly in the ultraviolet region) meant that addition of spectroscopic analysis was not easy. Hydroxide ions could be removed by using a micro-membrane chemical suppressor device, but this introduced new problems, as the acetate typically used as a ‘pushing agent’ for eluting capsular saccharides is converted by the suppressor into highly absorbent acetic acid.
It is an object of the invention to provide further and improved methods and systems for performing anion chromatography characterisation of saccharides. In particular, it is an object to overcome the difficulties in spectroscopic analysis of eluates that arise from the presence of either hydroxide ions or, after conversion of hydroxide ions, acetic acid.